especially if the Fan is the Heatsink Fan over Processor. You could burn out the CPU, then there may be a fan on your AGP Graphics Card and again rapid replacement is needed
Rear case cooling fan may be there and it could be that one.
Any fans giving of that sort of sound as you decribe is a bearing wear in the centre housing of the fan. A Fan Check & Replacemnt is needed either by yourself or qualified person.
Leaving it may cause perminant damage at some stage.